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ETA CETa Exam Common Formulas Ohm’s Law Potential Divider As the name says, we divide the potential or reduce the voltage in a circuit with help of potential divider. E2 R Vout = Vin * R2 / (R1 + R2) ExI Current Divider It is used to redirect current flowing in a circuit. E2 P Iout = Iin * R1 / (R1 + R2) 2 I xR P R Balanced Wheatstone Bridge A bridge used to measure resistances. PxR E (R1 / R2) = (R3 / R4) Voltage gain in decibels Gain dB = 20 log (Vout / Vin) Ratio of 2 power levels in decibels Gain dB = 20 log (Vout / Vin) Resonant frequency FR = .159 / LC P = I * E, the power being dissipated by the resistor is a product of the current and the applied voltage. Resistors in series R = R1 + R2 + R3... Resistors in parallel 1 / R = (1 / R1) + (1 / R2) + (1 / R3)... The resistance of a conductor at a temperature, t, is given by the equation: Rt = R0(1 + αt + b t2 + y t3) where α, b, y are constants and R0 is the resistance at 0°C. Note that b & y are very small hence they can be neglected. Therefore above equation simplifies to: Rt = R0(1 + αt) where α = temperature coefficient of resistance. Inductors connected in series L = L1 + L2 + L3 +... Inductors connected in parallel 1 / L = (1 / L1) + (1 / L2)... Reactance of inductors XL = 2 * * f * L where XL is reactance, f is frequency, and L is inductance P I E I I I x R P/R P I2 P E E R E = Voltage I = Current P = Power R = Resistance Current flowing in a Capacitor The current flowing in a capacitor is proportional to the product of the capacitance, C, and the rate of change of applied voltage. i = C × (rate of change of voltage[d * V / d * t]) How to Compute Charge or Quantity of Electricity Q = C* V where Q is the charge (in coulombs), C is the capacitance (in farads), and V is the potential difference (in volts). Energy Storage in a Capacitor W = ½ C * V2 where W is the energy (in Joules), C is the capacitance (in Farads), and V is the potential difference (in Volts). Capacitors connected in parallel C = C1 + C2 + C3 +... Capacitors connected in series 1 / C = (1 / C1) + (1 / C2)... Reactance of capacitors XC = 1 / (2 * * f * C)